Formulae within Pivot Table Report?

"Cannot enter a formula for an item or field name in a PivotTable report."

I've been getting this error message a few times now and I cant seem to work a way around it. I'd like to automate this somehow so a selection from a drop down menu I make on another sheet replaces the value in the pivot table. In the pivot table, the cell would be different states, so you can see whatever states data you want.

This is happening somewhere else in the workbook so I know it's possible, I just cant seem to figure it out. I can clarify anything that seems confusing. Thank you.